The DTR-process initially intended for office copying purposes has found now wide application in the graphic art field, more particularly in the production of screened prints from continuous tone originals.
For the latter purpose it is particularly important that the processing characteristics remain steady for a large set of prints and that the gradation, optical density (transmission density in the case of film material and reflection density in the case of opaque material e.g. paper material) and the neutrality of the colour tone (black) of the screen dots in the screened prints satisfy graphic art standards which are particularly high compared with normal copying.
The principles of the silver complex diffusion transfer reversal process have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an image-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values with respect to the silver image obtained in the exposed photographic material.
The developing agent or a mixture of developing agents can be present in an alkaline processing solution and/or in the photographic silver halide emulsion layer material. In case the developing agent or a mixture of developing agents is contained in the photographic silver halide emulsion material, the processing solution can be merely an aqueous alkaline solution that initiates and activates the development.
Suitable developing agents for the exposed silver halide are e.g. hydroquinone-type and 1-phenyl-3-pyrazolidone-type developing agents as well as p-monomethylaminophenol.
The silver halide solvent, mostly sodium thiosulphate, may be supplied from the non-light-sensitive image-receiving element as mentioned above, but it is normally at least partly already present in the alkaline processing solution.
The alkaline processing solution usually contains sufficient alkaline substances to bring the pH above 10, e.g. sodium hydroxide, sodium carbonate and a great many other compounds which can raise the pH, for example borax, tertiary sodium phosphate, lithium hydroxide and amines, particularly alkanolamines.
The use of alkanolamines in processing solutions for DTR is disclosed in U.S. Pat. No. 4,568,634. However the there disclosed processing solutions contain phosphate salts in substantial amounts which is undesirable for ecological reasons. It is further known that phosphates may inhibit the swelling of gelatin, reduce the gradation and increase the stain in the image some time after the preparation of the processing solution.
Processing solutions for DTR substantially free of phosphate are disclosed in U.S. Pat. No. 4,632,896 and the European patent applications EP 397925, EP 397926, EP 397927 and EP 398435. However the processing characteristics of these processing solutions do not remain steady over a sufficient number of prints due to absorption of CO.sub.2 from the air. Furthermore, although the processing characteristics of these fresh processing solutions are good it is still desirable to also improve these characteristics.